The completed sequences of gene and protein for Histidinol dehydrogenase from S. typhimurium will be exploited as the base-point of a study of structure, function, and genetic relationships. Mutant enzymes will be created with altered catalytic properties: alterations will be characterized by DNA sequencing, facilitated by a close-linked coorelation between physical and genetic maps: changes in enzyme properties will be related to the structural changes. Mutants studied will include those resulting in thermolabile enzymes, altered Km for histidinol, and pairs of mutants which act to correct each other's defect (complementation). The long-range goal is to understand these effects in terms of protein structure revealed by x-ray crystallography.